1. Field
The present invention generally relates to an apparatus, system, and method for interference cancellation in a receiver. More specifically, one feature of the invention may remove intermodulation interference, caused by transmitted signals or other signals that leak onto a receiver, at baseband.
2. Background
Many communication systems support full duplex transmissions where information (e.g., voice, digital data, video, etc.) can be sent in both directions of a communication link at the same time. This permits, for instance, two parties on different sides of a communication link to talk at the same time. Conventional full duplex communication devices transmit on one frequency and receive on a different frequency. In many communication devices (e.g., wireless telephones, cellular telephones, telephones, base stations, etc.) a transmitter and receiver share a single antenna (via a duplexer or switch), and the signal transmitted is stronger than the signal received. That is, since a transmitted signal often has to travel significant distances to reach its destination (e.g., receiving device, base station, relay station, etc.), it is likely to have significant attenuation by the time it reaches the receiving device. The transmitted signal also has to have sufficient power to be distinguishable over noise and interference perceivable at its destination.
In full duplex communication systems having a transceiver (i.e., combination of transmitter and receiver), the signal being transmitted is amplified by one or more power amplifiers (PA) prior to transmission. This amplified transmitted signal often leaks into the receiver's path due to the finite attenuation of a duplexer circuit, thus interfering with the received signal. Additionally, the receiver's path often includes interference (called “jammer signals”), which may be substantially stronger in amplitude than the desired or intended receive Rx signal.
With an ideal, perfectly linear radio frequency (RF) down-conversion chain, the transmission signal crossover would not present a problem because the transmitted and received signals occupy different frequency bands, separated by what is called the duplex frequency (e.g., 45 MHz for cellular band code-division multiple access (CDMA)). Since the stronger transmitted signal is, for instance, 45 MHz away from the received signal, a perfectly linear down-conversion chain would maintain this separation all the way to zero-frequency (baseband), where the interfering transmitted signal can be filtered out.
FIG. 1 is a block diagram illustrating a typical transmitter 102/receiver 104 arrangement for a full-duplex communication device, such as a CDMA communication device. A duplexer 106 couples the transmitter 102 and receiver 104 to permit signals to be transmitted and received through an antenna 108. The transmitter 102 typically includes a signal source 114 that generates a baseband signal (TxBB) that is up-converted by a mixer 115 with a transmission carrier frequency ωT 116. The signal from the mixer 115 is then amplified by a power amplifier 118, and transmitted via the duplexer 106 and antenna 108. The receiver 104 typically includes a low noise amplifier (LNA) 120 that receives a composite receive (Rx) signal 119 from the duplexer 106. A receiver mixer 121 demodulates the signal from the receiver carrier frequency ωR 122 to baseband and then passes it to a low pass filter 124, an analog-to-digital (A/D) converter 126, a digital low pass filter 127, and to a receiving device 128. The composite Rx signal 119 may include the leaked Tx signal 110, a jammer signal, and the intended Rx signal 112.
The Tx signal 111, which is typically much stronger than the intended Rx signal 112, often leaks through the duplexer 106 into the receiver's low noise amplifier (LNA) 120. In a typical situation for code division multiple access (CDMA), for instance, the Tx signal's 111 maximum power is approximately +28 dBm, and the duplexer's 106 transmitter-to-receiver isolation is approximately 60 dB. This means that the leaked Tx signal's 110 interfering power at the receiver's amplifier 120 is approximately −32 dBm, which is much stronger than the intended Rx signal 112 which can be as low as −100 dBm or less. If the receiver mixer 121 were perfectly linear, the frequency separation of the leaked Tx signal 110 and intended Rx signal 112 would be maintained (e.g., 45 MHz of separation), and the low-pass filter (LPF) 124 would eliminate the leaked Tx signal 110.
A circuit or component is “linear” when it applies a linear transfer function (i.e., a function which, if the input is scaled by a certain factor, causes the output to also be scaled by an identical factor) to input signals regardless of the input signals' characteristics. For instance, a component is free from nonlinearity if it applies the same scaling factor to all input signals regardless of input signal amplitude. One effect of nonlinear components is that the input signal's frequency bandwidth is broadened. For example, an input signal that initially occupies a narrow frequency bandwidth ends up occupying a wider range of frequencies. Therefore, circuits with nonlinearity often increase the bandwidth of modulated input signals.
As a result of the nonlinearity of the mixer 121, the leaked Tx signal 110 is squared and occupies the same region around baseband (i.e., 0 Hz) as the down-converted (weaker) intended Rx signal 112. Thus, the traditional receiver 104 cannot adequately filter out the leaked Tx signal 110, making it more difficult to recognize received signals.
The composite Rx signal 119 also includes one or more jammer signals that may or may not be modulated in a similar way as the leaked Tx signal 110. For example, in various implementations a jammer signal may be a tone or a modulated signal (e.g., possibly of some other wireless communications standard or network) with a center frequency close to the Rx center frequency ωR. The modulation of the transmit signal transfers (crosses) to the jammer signal. Due to the proximity of the receiver frequency ωR to typical interference (i.e., jammer) frequencies, the frequency spectrum of the jammer signal can overlap onto the receive frequency ωR. Thus, the stronger jammer signal may overshadow the intended receive Rx signal 112, making it difficult to discern.
One way to reduce unwanted signals at the receiver 104 is to filter the amplifier 120 output to remove unwanted signals, for example, by means of a sharp RF filter, often in practice an external SAW filter 123, between LNA 120 and mixer 121. However, the external SAW filter 123 is costly and is only effective in removing the interfering Tx signal 110, not the jammer. Since the frequency spectrum of the jammer signal may occupy the same frequency space as the intended Rx signal 112, such filtering would also filter-out the intended Rx signal as well. Another technique is to filter the leaked Tx signal 110 and jammer signal from the receive path before amplification. This technique is not entirely adequate because (a) the leaked Tx signal and jammer signal may be too close to the intended Rx signal 112 to filter, and (b) large and expensive duplexers and filters may be necessary.
Thus, traditional receivers 104 typically use external SAW filters 123 in combination with a highly linear mixer to prevent leaking and filter out leaked transmission signals. However, external SAW filters are costly, and the highly linear mixers increase power consumption.